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Pumps - Specific Speed

Characterizing of impeller types in pumps in a unique and coherent manner.

Specific Speed is a number characterizing the type of impeller in pump in a unique and coherent manner. Specific speed is determined independent of the pump size and can be useful when comparing different pump designs.

• Specific Speed identifies the geometrically similarity of pumps

Specific speed can be expressed as

Ns = n q1/2 / h3/4                              (1)

where

Ns = specific speed

n = pump shaft rotational speed (rpm)

q = flow rate (m3/h, l/s, l/min, m3/min, US gpm, British gpm) at Best Efficiency Point (BEP)

h = head rise (m, ft)

Online Specific Speed Pump Calculator

The calculator below can used to calculate the specific speed:

Note! When comparing pumps and their documentation - be aware of the units used.

Typical values for specific speed - Ns - for different designs in US units (US gpm, ft)

• radial flow pump design - 500 < Ns < 4000 - typical for centrifugal impeller pumps with radial vanes - double and single suction. Francis vane impellers in the upper range
• mixed flow pump design - 2000 < Ns < 8000 - more typical for mixed impeller single suction pumps
• axial flow pump design - 7000 < Ns < 20000 - typical for propellers and axial fans

Convert Specific Speed between US units (gpm) and Metric units (m3/h)

• Ns(US gpm, ft) = 0.861 Ns(m3/h, m)
• Ns(British gpm, ft) = 0.787 Ns(m3/h, m)

when

• 1 US gpm = 0.2271 m3/h
• 1 British gpm = 0.2728  m3/h
• 1 ft = 0.3048 m

Example - Specific Speed for a Pump with different units

A pump has the capacity 1500 US gal/min (1249 British gpm, 340 m3/h, 5667 liters/min) at 100 ft(30.5 m) of head and is rotating at 1760 rev/min.

US gpm, ft

Ns(US gpm, ft) = (1760 rev/min) (1500 gal/min)1/2 / (100 ft)3/4

= 2156

British gpm, ft

Ns(British gpm, ft) = (1760 rev/min) (1249 gal/min)1/2 / (100 ft)3/4

= 1967

m3/h, m

Ns(m3/h, m) = (1760 rev/min) (340 m3/h)1/2 / (30.5 m)3/4

= 2500

liter/min, m

Ns(liters/min, m) = (1760 rev/min) (5667 liters/min)1/2 / (30.5 m)3/4

= 10209

Specific Speed Hydro-power Turbines

Specific speed is also used when evaluating hydro-power turbines. Typical range of turbine specific speeds:

• Francis turbine : 70 - 500
• Kaplan or propeller turbine : 100 - 350
• Pelton, Turgo or Cross-flow turbines : 10 - 80

Related Topics

• Pumps

Design of pumping systems and pipelines. With centrifugal pumps, displacement pumps, cavitation, fluid viscosity, head and pressure, power consumption and more.

Related Documents

• BEP - the Best Efficiency Point of a Pump

BEP is where the pump is most efficient.
• Centrifugal Pumps

An introduction to Centrifugal Pumps.
• Centrifugal Pumps - Capacity Modulation

Modulating pumps to adapt capacities to variable process demands.
• Centrifugal Pumps - Minimum Flow

Minimum continuous flow to prevent flashing in centrifugal pumps.
• Centrifugal Pumps - Stable and Unstable Flow

The characteristics of stable and unstable flow.
• Efficiency in Pumps or Fans

The overall pump and fan efficiency is the ratio power gained by the fluid to the shaft power supplied.
• Fans - Calculate Air and Brake Horsepower

AHP - Air Horse Power and BHP - Brake Horse Power.
• Pumps - Affinity Laws

Turbo machines affinity laws can be used to calculate volume capacity, head or power consumption in centrifugal pumps when changing speed or wheel diameters.
• Pumps - Suction Specific Speed

Suction Specific Speed can be used to determine stable and reliable operations for pumps with max efficiency without cavitation.
• Pumps, Fans and Turbines - Horsepower

British Horse Power as used for pumps, fans and turbines - and how to convert to other units.
• Turbo Machines - Specific Work done by Pumps, Compressors or Fans

Calculate specific work done by pumps, fans, compressors or turbines.

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Citation

• The Engineering ToolBox (2004). Pumps - Specific Speed. [online] Available at: https://www.engineeringtoolbox.com/specific-speed-pump-fan-d_637.html [Accessed Day Month Year].

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7.8.15

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